**2. Materials and methods**

78 New Approaches to the Study of Marine Mammals

marine mammalian food-chains.

There are several measures that have been used to express the degree of biomagnification. The simplest measure is the Biomagnification Factor (BMF), which is described as the ratio of the chemical concentrations in the organism (*C*B) and the diet of the organism (*C*D), i.e., BMF = *C*B/*C*D, where the chemical are usually expressed in units of mass of chemical per kg of the organism (in wet weight or in a lipid basis) and mass chemical per kg of food (in wet weight or in a lipid basis) [6]. Biomagnification of organic contaminants and foraging preferences in aquatic and marine food webs can also be investigated using stable nitrogen isotope as biomarkers of trophic level [15-20]. Stable isotope analysis (SIA) has emerged as a tool in foraging ecology/habitat use, physiology and ecotoxicology, and is applied widely to study marine mammal ecology [21]. Stable nitrogen isotope analysis is a known well established technique for assessing predator–prey interactions and organism trophic levels (TL) in food webs [22-25]. Specifically,*δ*15*N*, the concentration ratio of 15*N*/14*N*, expressed relative to a standard (i.e., atmospheric *N*2), has been shown to increase with increasing trophic level due to the preferential excretion of the lighter nitrogen isotope [26]. Likewise, carbon isotope signatures (*δ*13C) provide information on habitat use and general sources of

diet of organisms, i.e., marine/freshwater, coastal/oceanic, pelagic/benthic [27].

Studies of the biomagnification and food web transport of POPs in tropical systems such as remote islands around the equatorial Pacific Ocean are lacking. Due to the remoteness and isolation of the Galapagos Islands relative to other better studied geographical areas, the Galapagos Island food web offers a unique opportunity to undertake research related to the transport, bioaccumulative nature and biomagnification of globally distributed contaminants in tropical environments at the ecosystem level. The low population levels and generally good environmental control and management practices on the islands ensures that local pollutant sources are in most cases insignificant compared to global sources. These conditions provide a unique mesocosm to study the behaviour of global pollutants in

The Galapagos sea lion(*Zalophus wollebaeki*) is an endemic marine mammal residing year round in the islands and exhibiting a high degree of dietary plasticity, consuming several groups of fish prey (99% of the diet). The Galapagos sea lion diet includes Cupleidae (thread herrings and sardines), Engraulidae (anchovies), Carangidae (bigeye scads), Serranidae (groupers, whitespotted sand bass or camotillo), Myctophidae (lantern fishes), Mugilidae (mullets) and Chlorophtalmidae fishes, and a low proportion of squid, as reported in the existing literature [28-31]. Although the information about diet and trophic level is limited for sea lions at several rookeries in the Galapagos Islands, it is known that the dietary preferences of Galapagos sea lions are also a function of the local variation in prey availability and regional climate-oceanic variability such as the El Niño events, when sea lions can switch their diet composition to more abundant fish items [30, 32, 33]. The Galapagos sea lion has been recognized as a key species for the functioning and health of the marine ecosystem of the islands under the environmental management action plan of the Galapagos Marine Reserve (GMR) [33]. Because of its high trophic position, relative abundance in the islands and non-migratory behaviour, Galapagos sea lions can serve as
